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loop-util: don't reuse partition fd when partscan needed

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Some devices (e.g. android phones running pmOS) cannot have their OEM
partition table altered without breaking the firmware, so the distros's
partitions live inside a nested GPT carved into one of the OEM
partitions. Exposing these subpartitions requires wrapping the outer
partition in a loop device with partscan enabled, since the kernel does
not go into nested partition tables.

systemd already detects this case in udev-builtin-blkid
(ID_PART_GPT_AUTO_ROOT_DISK_NEEDS_LOOP) and acts on with
systemd-loop@.service, but this fails towards the end.
loop_device_make_internal has an optimization where if the input is
already a block device with a matching sector size, it skips creating
a loop and just hands back the original fd. That's fine for whole disks
but wrong for partitions, which don't support partscan, so this causes
dissect_image to fail with EPROTONOSUPPORT.

This patch changes the behavior to only take the shortcut when the input
is a whole disk, or when partscan was not requested.

Co-Authored-By: Clayton Craft <clayton@craftyguy.net>
This commit is contained in:
Clayton Craft
2026-04-28 15:25:07 +01:00
committed by Luca Boccassi
parent d0f726ed8f
commit 47d408163b
2 changed files with 73 additions and 7 deletions
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src/test/test-loop-util.c
+33
View File
@@ -538,4 +538,37 @@ TEST(sector_size_mismatch) {
loop = loop_device_unref(loop);
}
TEST(partscan_required) {
_cleanup_(loop_device_unrefp) LoopDevice *block_loop = NULL, *loop = NULL;
_cleanup_close_ int fd = -EBADF;
if (have_effective_cap(CAP_SYS_ADMIN) <= 0) {
log_tests_skipped("not running privileged");
return;
}
if (detect_container() != 0 || running_in_chroot() != 0) {
log_tests_skipped("Test not supported in a container/chroot, requires udev/uevent notifications");
return;
}
ASSERT_OK(make_test_image(&fd));
/* Set up a backing loop device without LO_FLAGS_PARTSCAN. */
ASSERT_OK(loop_device_make(fd, O_RDWR, 0, UINT64_MAX, 0, 0, LOCK_EX, &block_loop));
ASSERT_TRUE(block_loop->created);
ASSERT_OK(loop_device_flock(block_loop, LOCK_SH));
/* Without LO_FLAGS_PARTSCAN: shortcut should be taken (reuse existing loop). */
ASSERT_OK(loop_device_make(block_loop->fd, O_RDWR, 0, UINT64_MAX, 0, 0, LOCK_SH, &loop));
ASSERT_FALSE(loop->created);
loop = loop_device_unref(loop);
/* With LO_FLAGS_PARTSCAN: backing loop has partscan disabled, so a new loop device with
* partscan must be created. */
ASSERT_OK(loop_device_make(block_loop->fd, O_RDWR, 0, UINT64_MAX, 0, LO_FLAGS_PARTSCAN, LOCK_SH, &loop));
ASSERT_TRUE(loop->created);
loop = loop_device_unref(loop);
}
DEFINE_TEST_MAIN_WITH_INTRO(LOG_DEBUG, intro);